The present invention relates to a magnetic recording medium excellent in coating film smoothness, electromagnetic characteristics and running durability.
Magnetic recording media are widely employed in recording tapes, videotapes, computer tapes, disks, and the like. Magnetic recording media are becoming denser and the recording wavelengths are becoming shorter each year. There is also an examination underway as to whether the recording method is changed from analog to digital.
With the high densification of magnetic recording media, a demand for greater coating film smoothness has come and magnetic materials tend to be in the form of microparticles. Kneading technology is being employed to exert great shearing force on magnetic and nonmagnetic materials to achieve a high degree of dispersion. In particular, since microgranular ferromagnetic material with a major axis length equal to or less than 100 nm tends to aggregate, adequate dispersion properties cannot be achieved without the use of kneading technology, and a kneading step is required.
To prevent combustion due to rapid oxidation of ferromagnetic material during kneading, the process is conducted in an oxygen-free environment achieved by means of a nitrogen purge and the like (Japanese Examined Patent Publication (KOKOKU) Heisei No. 7-43819 and Japanese Unexamined Patent Publication (KOKAI)) Heisei No. 11-213379).
To achieve good coating film smoothness, the technique of employing vinyl chloride copolymer having radiation-curing functional groups in a magnetic layer having microgranular magnetic material has been proposed (Japanese Examined Patent Publication (KOKOKU) Heisei Nos. 1-25141 and 5-74621). However, when vinyl chloride copolymer having radiation-curing functional groups is employed, there are problems during the step of kneading in an oxygen-free environment in that radicals are produced by heat from shearing and a radical polymerization reaction sometimes causes the vinyl chloride copolymer to crosslink, resulting in decreased dispersion of the magnetic material and a coating liquid of high viscosity that is unsuitable for coating.
Further, when vinyl chloride copolymer having radiation-curing functional groups is employed in a system with nonmagnetic powder, since kneading can be conducted in air, any radicals that are produced are eliminated by oxygen and no reaction occurs such as that during kneading of ferromagnetic powder. However, in a magnetic recording medium in which an upper magnetic layer is sequentially coated over a nonmagnetic lower layer employing vinyl chloride copolymer having radiation-curing functional groups that has been cured with radiation (Japanese Unexamined Patent Publication (KOKAI) Heisei No. 1-213379), the durability of the magnetic layer is inadequate.
Accordingly, it is an object of the present invention to provide a magnetic recording medium having excellent coating film smoothness, electromagnetic characteristics and running durability.
The present inventors conducted further studies, resulting in finding that a magnetic recording medium comprising a smooth magnetic layer in which extremely microparticle magnetic material, that is, an acicular magnetic material having a major axis length ranging from 20 to 100 nm is dispersed well and achieving high electromagnetic characteristics and high running durability can be obtained by;
simultaneously multilayer coating a nonmagnetic layer and a magnetic layer, wherein the nonmagnetic layer comprises vinyl chloride copolymer with a molecular weight of 10,000 to 100,000 having a radiation-curing functional group as a binder component, and the magnetic layer does not comprise a binder component comprising a radiation-curing functional group, and
employing a certain type irradiation cross-linking cross-linker. Thus, the present invention was achieved.
The present invention relates to a magnetic recording medium comprising a nonmagnetic layer comprising a nonmagnetic powder and a binder and at least one magnetic layer comprising a magnetic material and a binder in this order, wherein
said magnetic material is an acicular ferromagnetic material having a major axis length ranging from 20 to 100 nm,
said nonmagnetic layer and at least a magnetic layer adjacent to said nonmagnetic layer are obtained by simultaneously multilayer coating a nonmagnetic layer coating liquid and a magnetic layer coating liquid and exposing to radiation,
the binder comprised in said nonmagnetic layer coating liquid comprises a vinyl chloride copolymer having a molecular weight ranging from 10,000 to 100,000 and a radiation-curing functional group,
said magnetic layer coating liquid does not comprise a binder component having a radiation-curing functional group,
said nonmagnetic layer coating liquid and/or said magnetic layer coating liquid comprise a compound having a molecular weight ranging from 200 to 2000 and a radiation-curing functional group with two or more functions, and
said magnetic layer adjacent to the nonmagnetic layer has a thickness equal to or less than 0.5 xcexcm.